Mechanism Of Deep Earthquakes Research Articles

Unsupervised machine learning reveals slab hydration variations from deep earthquake distributions beneath the northwest Pacific

Although transformational faulting in the rim of the metastable olivine wedge is hypothesized as a triggering mechanism of deep-focus earthquakes, there is no direct evidence of such rim. Variations of the b value – slope of the Gutenberg-Richter distribution – have been used to decipher triggering and rupture mechanisms of deep earthquakes. However, detection limits prevent full understanding of these mechanisms. Using the Japan Meteorological Agency catalog, we estimate b values of deep earthquakes in the northwestern Pacific Plate, clustered in four regions with unsupervised machine learning. The b-value analysis of Honshu and Izu deep seismicity reveals a kink at magnitude 3.7–3.8, where the b value abruptly changes from 1.4–1.7 to 0.6–0.7. The anomalously high b values for small earthquakes highlight enhanced transformational faulting, likely catalyzed by deep hydrous defects coinciding with the unstable rim of the metastable olivine wedge, the thickness of which we estimate at sim1 km.

Plastic Faulting in Ice

AbstractPlastic faulting is a brittle‐like failure phenomenon exhibited by water ice and several other rock types under confinement. It is suspected to be the mechanism of deep earthquakes and extreme cases of shear localization in shallow rocks. Unlike ordinary Coulombic failure, plastic faulting is characterized by a pressure‐independent failure strength and fault plane oriented 45° to maximum principal stress. To research the question of how the instability initiates, we conducted over 50 constant‐displacement‐rate experiments on polycrystalline ice (phases Ih and II) near the brittle‐to‐ductile (B‐D) transition, at confining pressures P = 0–300 MPa, applied strain rates = 5 × 10−5 – 7 × 10−3 s−1, temperatures T = 105–233 K, and mean grain sizes d = 0.25–1.18 mm. We find that (1) the width of the B‐D transition in variable space is vanishingly narrow, to the point of appearing as a crossover, (2) a plastic fault plane, once formed, is not a zone of subsequent weakness, (3) distributed ice I→II phase transformation in small amounts (<1 vol%) shows no causal relationship to subsequent failure, and (4) plastic faulting also occurs in ice II. We hypothesize that the elusive nucleating “trigger” parallels that of metals and ceramics undergoing severe plastic deformation, wherein transient local structural rearrangement occurs, in turn causing material strength to drop to a level sufficiently low, in a volume sufficiently large, that adiabatic instability is nucleated. Our results do not require and often are inconsistent with phase transformation. Plastic faulting may therefore be available to all solids undergoing severe deformation, and its appearance in so few is simply the result of insufficiently extreme conditions.

Dominant role of temperature on deep earthquake mechanics for the Tonga slab near the bottom of the upper mantle

The effects of temperature on the mechanics of deep earthquakes are investigated based on detailed seismic images of a horizontal portion of the Tonga slab (19°S to 22°S). The hypocenter distribution of deep earthquakes and tomographic models have shown that the Tonga slab in this region stagnates laterally around the upper- and lower-mantle boundary. We analyze data from seismic networks in the United States and Japan from the deepest earthquakes that occurred in the stagnating part of the slab (focal depths 658 to 678 km). We observe a clear arrival approximately 8 s after the direct P wave and find that it is an S-to-P converted wave emitted upward from the deepest earthquake foci and reflected downward by a horizontal interface located nearly 30 km above the foci. The S wave speed drops upward across the interface by a few percent, so the interface is best interpreted as the former oceanic Moho in the stagnating Tonga slab. A numerical model of the thermal structure of the Tonga slab is constructed based upon the slab geometry, with the dynamics of the slab motion being incorporated. We find that if the deepest earthquakes occur in the coldest part of the slab, the vertical distance between the foci and the Moho should be approximately 30 km, which agrees well with the observations. The temperature around the foci is determined to be 750°C. We suggest that deep earthquakes are stringently restricted below this temperature, near the bottom of the upper mantle.

Specific features of the appearance of seismotectonic deformations during the genesis of March 11, 2011, Tohoku earthquake source

The results of investigating the preparation zone of the catastrophic earthquake of March 11, 2011, near the eastern coast of Honshu Island are presented. A retrospective analysis of the focal mechanisms of deep earthquakes in the western part of the Pacific Ocean made it possible to reveal their influence on the zone of the Tohoku earthquake source. The preparation of this earthquake is also investigated within an analysis of the seismotectonic deformation processes at the regional level, which includes an estimation of the temporal trends of some key parameters, as well as the detection of their variations and the spatial confinement of anomalies to the source zone.

Metastable olivine wedge in the subducting Pacific slab and its relation to deep earthquakes

A metastable olivine wedge (MOW) is detected in the subducting Pacific slab under the Japan Sea by applying a forward-modeling approach with 3-D ray tracing to high-quality arrival-time data from 78 well-located deep earthquakes. The data were recorded by the dense and high-quality seismic networks on the Japan Islands. The double-difference earthquake location technique is improved and used to relocate the 78 deep earthquakes precisely. Our results show that the MOW does exist in the Pacific slab and it has a P-wave velocity 3% lower than the normal mantle or 4% lower than the surrounding slab velocity. The top and bottom depths of the MOW are 400 and 570 km, respectively, and the MOW is located in the central portion of the slab. After precise relocation, all the 78 deep earthquakes are found to occur within the MOW or along its edges within the slab. The cut-off depth of deep seismicity under the Japan Sea is consistent with the estimated MOW bottom depth. Our results favor the transformational faulting as the mechanism of deep earthquakes in the Pacific slab under the Japan Sea.

A general test of the hypothesis that transformation-induced faulting cannot occur in the lower mantle

Transformation-induced faulting, which has been proposed as a mechanism for deep-focus (>300 km) earthquakes, requires a thermal runaway associated with an exothermic chemical reaction. This shearing instability has been demonstrated for several polymorphic phase transformations including the olivine → wadsleyite and olivine → ringwoodite reactions. However, it has not been experimentally tested whether a strongly exothermic decomposition reaction can also lead to this instability, as would be the case if olivine were to be carried metastably into the lower mantle and break down to magnesium silicate perovskite + magnesiowüstite. Here we report experiments designed to test this possibility using the proxy reaction albite → jadeite + coesite, a highly exothermic reaction with a large ΔV. We find no mechanical or microstructural evidence for transformation-induced faulting during this reaction. Applying these results to the reaction olivine (ol) → perovskite (pv) + mw potentially occurring in subducting oceanic lithosphere at the top of the lower mantle suggests that this breakdown reaction cannot trigger earthquakes. Thus, transformation-induced faulting as a mechanism of deep earthquakes is consistent with the abrupt cessation of earthquakes at the base of the mantle transition zone, but only if subduction zones are colder than currently proposed. We discuss other reasons for termination of earthquakes at ∼700 km and find them also difficult to reconcile with experimental evidence or natural observations.

Seismic evidence for a metastable olivine wedge in the subducting Pacific slab under Japan Sea

We apply a forward-modeling approach to high-quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting Pacific slab beneath the Japan Sea. Our results show that a finger-like anomaly exists within the subducting Pacific slab below 400 km depth, which has a P-wave velocity 5% lower than the surrounding slab velocity (or 3% lower than that of the normal mantle), suggesting the existence of a metastable olivine wedge (MOW) in the slab. The MOW top and bottom depths are 400 and 560 km, respectively. The MOW is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary. At 560 km depth the MOW is located at about 25 km below the slab upper boundary. Most of the deep earthquakes are located in the MOW. Our results favor transformational faulting as the mechanism for deep earthquakes.

Shearing instabilities accompanying high-pressure phase transformations and the mechanics of deep earthquakes

Deep earthquakes have been a paradox since their discovery in the 1920s. The combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers, yet earthquakes occur continually in subduction zones to approximately 700 km. The expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self-organized failure analogous to, but different from, brittle failure. The only high-pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material. This "fluid" spontaneously forms mode I microcracks or microanticracks that self-organize via the elastic strain fields at their tips, leading to shear failure. Growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher-pressure polymorphs. In either case, fault propagation could be enhanced by shear heating, just as is sometimes the case with frictional sliding in the crust. Extensive seismological interrogation of the region of the Tonga subduction zone in the southwest Pacific Ocean provides evidence suggesting significant metastable olivine, with implication for its presence in other regions of deep seismicity. If metastable olivine is confirmed, either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast.

Evidence for a metastable olivine wedge inside the subducted Mariana slab

An anomalous seismic velocity structure is revealed inside the subducted Pacific plate at northern Mariana near the deepest earthquake foci. We observe anomalous later phases in the P wave coda for a small cluster of deep earthquakes and an anomalous differential P wave slowness between a few pairs of the earthquakes belonging to the cluster. We interpret these observations with a 5% low velocity wedge which is nearly 25 km wide at the depth of 590 km and is truncated at 630 km. The wedge is bounded by two steeply dipping (60°–70°) transition layers with the thickness of about 4 km. The highly localized seismic anomaly inside the slab represents a wedge of metastable olivine (α) embedded in dry ringwoodite (γ). Deep earthquakes occur across only one of the boundaries of the metastable wedge, with some of them outside the wedge. The mechanism of deep earthquakes is discussed based on the spatial pattern of seismicity relative to the wedge.

Characteristics of seismic radiation during the 1994 Bolivian earthquake and implications for rupture mechanisms

The PKP waves of the 1994 deep Bolivian earthquake (Mw = 8.3) recorded by the dense short‐period seismic array in Taiwan were used to make direct measurements of seismic source radiation. Employing array stacking technique, those data were used to reconstruct the time function of the source rupture. The well‐resolved frequency content of this source time function was from the period of 1 to 10 seconds and, to resolve the rupture properties, it provided source information at higher frequencies than that in previous studies from global seismic network data. Rupture time analysis identifies four asperities in the array observations, and the initial rupture time of each asperity correlates well with results from previous studies using global network data. The analysed results of this study also indicate that, unlike other asperities, the two major ones did not release as much high frequency energy. However, based on the limited broadband observations in Taiwan, the long‐period energy was normally released. This study concludes that this can be well explained by the lubrication of fault during the great earthquake. Several deep earthquake mechanisms have been discussed for this special rupture characteristic, and none may be fully ruled out.

Seismic discontinuities and subduction zones

The seismic discontinuities at 410 and 660 km depth in the mantle are generally believed to be due to phase transformations in its major component, olivine. An increasing observational base of short-period regional seismic network data supports this view. Here, we analyse cross-sections of discontinuity topography in the northwest Pacific subduction zones to infer the thermal and chemical state of the mantle in a subduction environment. Most of the data is from the southern part of the Izu-Bonin subduction zone. Penetration of the lower mantle by the steeply dipping subducting slab is clearly indicated by depression of the 660 km discontinuity. Elevation of the 410 km discontinuity in this region by up to 60 km implies a thermal anomaly of approximately 900–1000°C relative to the ambient mantle. Observation of the elevated 410 in the seismically active part of the slab indicates that the olivine → wadsleyite phase transformation occurs under essentially equilibrium conditions there, which argues against the transformational faulting hypothesis as the mechanism for deep-earthquakes. Upgoing P-wave reflections from the 410 km discontinuity indicate that it is sharp in the mantle far from subducting slabs. Reflection coefficient variability in and near slabs indicates either a chemical effect or the consequences of topographic focusing.

Strain pattern of the Southern Tyrrhenian slab from moment tensors of deep earthquakes: implications on the down-dip velocity

Seismic strain is analysed along the slab face of the Southern Tyrrhenian subduction zone using focal mechanisms of deep earthquakes which occurred in the period 1960-1998. Results show that the slab is mostly affected by down-dip shortening which strongly increases below 250 km depth. Extensional strain is mainly confined to the direction perpendicular to the slab face. The dominance of down-dip shortening and the minor along strike inplane extension implies thickening of the slab below 250 km depth. Assuming constant seismic efficiency along the slab, this strain pattern also implies a decrease of the down-dip velocity below 250 km depth. We also locate lower magnitude intermediate-depth and deep earthquakes using arrival times since 1985 available from the Italian seismic national network. These data show that the slab reaches the deeper part of the upper mantle, as suggested by the occurrence of a few ??600 km depth earthquakes, and that a large portion of the Tyrrhenian slab, between 100 and 250 km depth beneath the offshore of the Calabrian arc, is aseismic. Only a short part of the Tyrrhenian slab is seismically continuous from the top to the bottom. The lack of seismicity may indicate either that aseismic subduction is occurring or that the slab is detached from its upper part. Although the data are still inconclusive, they suggest that an aseismic subduction is the most appropriate interpretation, considering recent tomographic images of the slab and the results of this study, which agree well with the presence of a neutral down-dip stress zone, as also observed worldwide in deep slabs.

The 1996 June 17 Flores Sea and 1994 March 9 Fiji-Tonga earthquakes: source processes and deep earthquake mechanisms

Source parameters of the 1996 Flores Sea and 1994 Fiji–Tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad-band seismograph stations. Both events consisted of several subevents. Models to approximate the spatial and temporal extent of the source process include point sources, propagating point sources and a combination of these. For the Flores Sea event, rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of P-wave pulses, in agreement with the findings reported by other investigators. Our preferred model suggests bilateral rupture propagation. It consists of four point sources that have variable double-couple radiation patterns and source time histories, and explains well the large compensated linear vector dipole (CLVD) component inferred from the Harvard centroid moment tensor (CMT) solution. The main moment release in the Fiji–Tonga event lasted only about 15 s. Our best model consists of two point sources with a total moment release of 2.8 × 1020 N m. Rupture propagated subhorizontally from the nucleation point to the north. The termination of rupture was located about 40 km to the north of rupture initiation. The inferred velocity of moment release in the Flores Sea and Fiji–Tonga events was 3–5 km s-1, a value which is higher than that inferred for the great Bolivian earthquake of June 1994. Other derived source parameters (static stress drop, radiated seismic energy and maximum seismic efficiency) are also significantly different from those inferred for the 1994 Bolivian event, suggesting that deep earthquake processes do not follow an easily detectable common mechanism.

Fine structure of deep Wadati–Benioff zone in the Izu–Bonin region estimated from S-to-P converted phase

Mechanism of deep earthquakes is one of the most important and still unsolved problems in the field of solid geophysics. Determining precise location of the deep earthquakes can provide us with clues to approach this problem. In this study, we address the problem by estimating the separation distances between hypocenters of the deep-earthquakes and slab–asthenosphere interface. We use later arrivals immediately after initial P arrivals from the deep earthquake events in the Izu–Bonin region. We interpret these later arrivals as an sP phase which is converted S-to-P at the upper slab interface. Our results prove that the hypocenters underlie the subducting upper slab interface. Using time delays between P and sP phases, we find that the interface is located 20 km above the earthquake hypocenters. Moreover, our results also indicate that the almost all deep earthquake hypocenters are distributed within a thin zone about 7 km wide rather than being scattered throughout the slab. This observation fits a hypothesis that deep earthquakes are caused by transformational faulting of metastable olivine model.

Seismological evidence for the existence of anisotropic zone in the metastable wedge inside the subducting Izu‐Bonin Slab

The mechanism of deep earthquakes has been a puzzle since their discovery almost 70 years ago. Experimental observations at high‐pressure have reported faulting caused by spinel‐filled anti‐cracks in metastable olivine and suggesting that the anticrack mechanism is responsible for deep earthquakes. We compared the shear‐wave splitting of earthquakes from the upper and lower planes of the double seismic zone to detect anisotropy caused by the faultings inside the metastable wedge. We detected the anisotropic zone inside the metastable wedge at the depth of about 340–390 km, which corresponds to the depth range of a second peak on the histogram of earthquakes. The observed polarization direction, east‐west, is almost parallel to the direction of subduction, which can be explained by a view that the shear‐wave splitting is caused by the en echelon arrays of spinel‐filled anti‐cracks and anticrack‐associated faultings.

Effect of slab temperature on deep-earthquake aftershock productivity and magnitude–frequency relations

DEEP earthquakes generally show fewer aftershocks and a larger variability in magnitude–frequency relations than shallow earthquakes1–5. These characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes, which is still uncertain6–8. Here we show that systematic variations in aftershock productivity and magnitude–frequency relations are related to the temperature of the downgoing slab, as inferred from the vertical velocity and age of the subducting lithosphere. Deep earthquakes with substantial aftershock sequences are found only in colder slabs, and earthquakes in warmer slabs consistently show few aftershocks. Magnitude–frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure, with warmer slabs showing fewer small earthquakes (lower 'b-values'). The statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes, and not adequately explained by simple geometrical limitations on fault width.

Transformation-induced faulting requires an exothermic reaction and explains the cessation of earthquakes at the base of the mantle transition zone

In many subduction zones, earthquakes occur continuously to the bottom of the mantle transition zone and stop abruptly at the endothermic phase change marking the top of the lower mantle, suggesting cause and effect. New experiments on polycrystals undergoing the ilmenite ↔ perovskite transformation show that transformation-induced faulting occurs only in the exothermic direction of the reaction. The close structural and thermodynamic analogies between the transformation studied here in CdTiO3 and those expected in the silicates of the earth's mantle argue strongly that these results are relevant to transformations occurring in subducting lithosphere. Moreover, consideration of the thermodynamics of this asymmetry show that this result should be a general phenomenon. If so, it follows that faulting by this mechanism cannot occur in the lower mantle, providing a natural explanation for the termination of earthquakes just before 700 km depth. If a ‘wedge’ containing metastable olivine exists at depth in subduction zones and is necessary for deep earthquakes, our results remove the constraint that this ‘wedge’ must taper to zero where the deepest earthquakes occur. The characteristics of the large deep earthquakes of 1994 and their aftershocks show that deep earthquakes can propagate beyond the zone of previous hypocenters and trigger aftershocks there, providing important new information about the mechanism of deep earthquakes and pointing to the need for additional experiments.

The mechanics of deep earthquakes: H. W. Green II & H. Houston, Annual Review of Earth & Planetary Sciences, 23, 1995, pp 169–213

Deep-focus earthquakes have remained a puzzle for researchers since 1928 when they were first reliably identified by Kiyoo Wadati in Japan. At the time it was understood that earthquakes at these depths should not have been possible as ductile deformation should be dominant. It is now known that deep-focus earthquakes occur in the depth range of 300 to 700 km with two peaks in activity around 400 and 600 km, although this does vary among slabs. Investigations into the mechanism responsible for generating deep-focus earthquakes have concentrated on transformational faulting, a process whereby a metastable mineral undergoing a phase change can cause seismogenic failure via a thermally driven runaway mechanism. Various previous high-pressure high-temperature experiments have been conducted on the olivine-spinel transition in both the germanate analogue and the natural magnesium silicate systems. However there is contention over whether olivine could in fact survive metastably beyond 500 km, and there are other candidate phases that could be responsible. This thesis presents experiments involving MgGeO3 high-clinopyroxene - ilmenite transition as an analogue for MgSiO3 clinopyroxene-akimotoite transition. The P-T phase relations have been determined experimentally and from a Debeye fit to the thermal expansivity of the germanate clinopyroxene and ilmenite phases. Further experiments have demonstrated that, under a set of critical conditions, compression through the clinopyroxene-ilmenite phase boundary results in large acoustic emissions emanating from the sample volume, at pressures very close to the phase boundary. This implies that the natural transition may well be capable of being seismogenic within the Earth.

On the unstable propagation of a phase transition in a solid

To explore a possible mechanism of deep earthquakes, this paper analyzes the unstable propagation of a stress-induced phase transition which is initiated in a homogeneous stress field. This Stephen problem is formulated as an initial-value problem for the phase boundary, and the driving force of the boundary is computed by using the solution of the boundary-value problem for a partially transformed material. The propagation of the phase transition under uniform pressure is numerically simulated. It is shown that (1) under lower pressure, the transition is terminated at a certain size, but it can propagate unstably when an initially transformed region is sufficiently large; and (2) when the pressure attains a critical value, the propagation becomes unstable, and goes in a particular direction depending on the initial shape. These results confirm the possibility of the unstable propagation of phase transition, and provide a theoretical basis for the hypothesis that the phase transition of a mantle material can trigger a deep earthquake.

The Mechanics of Deep Earthquakes

The Mechanics of Deep Earthquakes